The present invention relates to rotary cam, expanding internal shoe drum brakes, and in particular, to a rotary cam assembly including a pair of cam members able to provide greater lift for longer shoe wear life.
Typically, expanding internal shoe drum brakes of the type which are in commercial use include actuators of the S-cam type, as may be seen by reference to U.S. Pat. No. 4,905,800, assigned to the assignee of the present invention and incorporated herein by reference. Although S-cam type brakes have been generally satisfactory, S-cams are inherently limited in the amount of "rise" of the cam followers, for a given "package size". More specifically, in a conventional drum brake, the radial distance between the brake shoe return spring and the inside diameter of the drum limits the size of the S-cam, and therefore, the rise or lift which can be transmitted to the cam followers and the brake shoes.
One possible way of overcoming the rise and space limitations inherent in an S-cam brake is to provide a separate cam for each brake shoe, in which case each cam will be substantially smaller in the radial direction. It is known, although not in widespread commercial usage, to provide a separate actuator for each brake shoe. See, for example, U.S. Pat. No. 3,249,182. Such an arrangement obviously adds substantial cost by requiring an additional actuator at each brake assembly, and in addition, there is the difficulty and expense of coordinating or synchronizing the application of the two actuators, so that both brake shoes are applied at the same time to achieve the type of braking action considered desirable by the vehicle operator. In fact, such coordination may be nearly impossible in a "leading-leading" arrangement of the type shown in the above cited patent.